The present invention concerns the field of aircraft structures. More particularly, but not exclusively, this invention concerns an aircraft structure comprising a structural component, and at least one reinforcing strap for reinforcing the structural component. The invention also concerns a method of reinforcing an aircraft structure, for example during repair of the structure or retrofit of an additional component to the structure. The invention also concerns a method of inspecting an aircraft structure.
It is sometimes desired to retrofit an additional component to an existing aircraft structure. For example, a wing tip device, such as a winglet, may be retrofitted to an existing aircraft wing. As part of the retrofit procedure, the aircraft structure needs to be reinforced. This is because the aircraft structure was not originally designed to include the additional component. In the wing tip device example, the wing box structure needs to be reinforced. It is also sometimes required to reinforce an existing aircraft structure as part of a repair procedure.
A typical wing box structure comprises an upper and a lower cover, comprising skin and spanwise extending stringers, and a front and a rear spar, extending spanwise along the wing box structure. The wing box structure also comprises ribs which extend chordwise across the wing box structure between the skins. The ribs are provided with mouseholes to allow the stringers to pass through. The lower cover allows access to the interior of the wing box structure via manholes, to allow inspection of the interior of the wing box structure. The stringers adjacent to the manholes are called manhole boundary stringers.
When a wing tip device is retrofitted to the outboard end of the wing, it is likely that the wing box structure requires reinforcement, including reinforcement of the manhole boundary stringers. However, due to the fact that the stringers must pass through the mouseholes, there is not much space for the reinforcement, which must be continuous. As a result, the stringers are often reinforced with a reinforcing strap made of high strength steel so that, for the same load, the strap is smaller than it would have been if it were made from aluminium alloy.
A side sectional view of part of a prior art aircraft outboard wing box structure, with a reinforced manhole boundary stringer, is shown in FIG. 1.
The wing box structure includes a lower cover structure 10 with a manhole boundary stringer 30 extending upwards from the cover structure 10 and extending in a spanwise longitudinal direction of the wing box structure. Extending in the chordwise direction of the wing box structure is a rib 20 with a mousehole 21 for allowing the stringer 30 to pass longitudinally through the rib 20. The stringer 30 comprises a web portion 31 which extends inwards from the cover structure 10 and an end portion which extends from the distal end of the web 31 portion in both chordwise directions to provide a chordwise extending flange 32. A reinforcing strap 40 is attached to one side of the web portion 31 of the stringer 30 and extends longitudinally along the stringer 30. The reinforcing strap 40 is made from high strength steel.
However, high strength steel has a low toughness compared to other materials. Hence, it has a lower resistance to crack propagation. This means that a small flaw in the material can develop into a crack in a shorter time frame than with other tougher materials. This can lead to a higher maintenance frequency to check for flaws and cracks in the reinforcing strap. Every maintenance operation causes downtime for the aircraft and costs money for the airline operating the aircraft.
In addition, it is important that a retrofit or repair procedure is as quick as possible to minimise downtime of the aircraft.
The present invention seeks to mitigate the above-mentioned problems. Alternatively or additionally, the present invention seeks to provide an improved aircraft structure that is less vulnerable to crack propagation, especially after reinforcement during a repair or retrofit procedure, for example.